This K01 award will provide Dr. Hong Van Tieu with the necessary training and mentorship to become an independent investigator in HIV epidemiology and prevention. Her short-term goals are to expand her experience in HIV epidemiology and prevention research among men who have sex with men (MSM), gain advanced skills in epidemiology, biostatistics, social network analysis, and qualitative research, and further the understanding of the influence of sexual networks on HIV and sexual risk behaviors among Black and White MSM. She will receive specialized training in the epidemiology of substance use and its relationship to HIV risk among MSM. Her multidisciplinary mentoring team has expertise in HIV and substance use epidemiology, prevention, and clinical research and strong track records of supporting junior faculty to become independently funded investigators. The New York Blood Center has a long history of conducting HIV epidemiology and behavioral and biomedical prevention research among MSM and is profoundly committed to the success of this K01 award. The dramatically disproportionate HIV infection rates among Black MSM in the U.S remain unexplained by differences in individual behavioral risk. One hypothesis is that the characteristics of sexual networks of Black MSM place them at increased risk of HIV compared to non-Black MSM. The purpose of this study is to directly compare the structural and compositional characteristics of sexual networks between Black and White MSM and to examine the relationship of their network characteristics to HIV infection and sexual risk behaviors using an innovative combination of qualitative and quantitative research methodologies. The proposed research will enroll a large sample of Black and White MSM (n=878) in New York City as a substudy of an NIH R01-funded study to evaluate the hypothesis that certain structural (e.g., network size, density, partner concurrency) and compositional (e.g., assortative and disassortative mixing, durability of relationships) characteristics of Black MSM sexual networks increase the men's risk of HIV infection. The specific aims of this study are the following: (1) to describe and compare the structural and compositional characteristics of sexual networks between Black and White MSM at the egocentric network level, (2) to compare the association of structural and compositional characteristics of sexual networks with HIV infection and sexual risk behaviors between Black and White MSM at the egocentric network level, and determine whether differences in sexual network characteristics explain the disparity in HIV prevalence between Black and White MSM, and (3) to explore in depth the contexts and meanings of sexual partnerships and networks by evaluating how Black and White MSM subjectively experience their sexual networks and perceive them to influence their sexual risk behaviors using qualitative research methods. A focus of the research is on drug and alcohol use within sexual networks and their relationship to HIV risk among MSM. This research will provide critically important information on the sexual networks of Black MSM to explain the alarming rates of HIV infection among this population and to inform development and testing of sexual network-based intervention strategies to reduce HIV acquisition and transmission risk. It will also serve as an excellent platform for the candidate to gain essential training in HIV epidemiology and prevention research and generate innovative ideas for her first R01 grant application.